Installation for manufacturing thermoplastic receptacles

ABSTRACT

The present invention relates to an installation for manufacturing a receptacle from a preform of thermoplastic material, the receptacle being suitable for being subsequently subjected, without significant deformation, to relatively severe temperature conditions encountered during hot-filling or pasteurization processes. The installation includes means for manufacturing a receptacle blank having a hot-shrunk body from the preform, and molding means for forming a final receptacle from the blank. The means for manufacturing a receptacle blank are adapted to form a blank whose shrunk body is longer than the body of the final receptacle to be obtained. The molding means are described in detail.

This is a divisional of Application No. 08/671,586 filed Jun. 28, 1996and now U.S. Pat. No. 5,785,921, which is the U.S. national stageapplication of PCT/FR94/01539, filed Dec. 27, 1994, the disclosures ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to improvements applied to methods and toinstallations serving to manufacture a receptacle, such as a bottle,from a preform of thermoplastic material, the receptacle being suitablefor being subjected subsequently, without significant deformation, torelatively severe temperature conditions encountered during processessuch as being filled with a hot liquid or having its contentspasteurized, in which method, a preform of thermoplastic material havingits body heated to a temperature not less than the softening temperatureof the thermoplastic material is molded by blowing or by stretch-blowingto form an intermediate receptacle of dimensions greater than those ofthe final receptacle that is to be obtained, said intermediatereceptacle subsequently being heat treated to obtain a blank having ahot-shrunk body, which blank is molded to form the final receptacle.

BACKGROUND OF THE INVENTION

Such a method and installation are known from document FR-A-2 658 119(EP-A-O 442 836).

Although that known method gives full satisfaction with respect to theadvantages it provides compared with earlier methods, it neverthelessturns out that receptacles obtained by implementing it have a bottomthat is subject to deformation during hot-filling, with said deformationconsisting in the bottom bulging outwardly, in particular in the zonewhere the bottom meets the side wall of the body. This bulging does nottake place in regular manner around the periphery of the receptacle, sothe shape of the bottom varies in uncontrolled manner, thereby causingthe receptacle to be unstable; the bottom of the receptacle is alsoweakened in the zone where bulging is manifest; finally, the volume ofthe receptacle is modified thereby.

It turns out that bulging of the bottom is a result of excessive localstretching of the wall of the receptacle in the above-mentioned zone andof the stresses induced during final blowing of the hot-shrunk blankbeing released because of the heating caused by filling with a hotliquid.

For manifest reasons of feasibility, the hot-shrunk blank has transverseand longitudinal dimensions that are smaller than those of the cavity 5of the mold in which final blowing takes place (see FIG. 1A of theaccompanying drawings): this ensures that the blank is not pinched whenthe mold is closed. It has been shown that during final blowing of saidblank under high pressure (e.g. in the range 3 to 4×10⁶ Pa), the topportion 6 of the body of the blank 3 is pressed in the first placeagainst the walls of the mold cavity to form the shoulder of the finalreceptacle; thereafter the blank is pressed against the walls of themold cavity in a manner that varies continuously from the shouldertowards the bottom, so that the bottom of the receptacle is formed last.Unfortunately, during this progressive molding process, forming of theshoulder followed by forming of the body of the receptacle means thattheir respective diameters are increased, and given that thethermoplastic material is not stretched from one region towards another,this is accompanied by a progressive decrease in the length of theblank. As a result, when it is formed, the not-yet formed bottom 7 ofthe blank is at a considerable distance from the bottom 8 of the moldcavity 5 (see FIG. 1B) so final forming of the bottom of the blank isaccompanied by considerable stretching of the material along two axes(both longitudinal and transversal), which stretching is caused byexpansion of the material: the receptacle 4 as finally obtained (seeFIG. 1C) thus presents considerably greater stretching in its now-formedbottom zone 9, thus having reduced thickness, and it also has inducedstresses which are greater than those in the remainder of thereceptacle, which stresses will subsequently be released during heatingof the kind caused by hot-filling.

Document EP-A-O 559 103 proposed a solution for remedying that drawbackand obtaining a receptacle having a mechanically strong bottom. However,that known method is designed for manufacturing receptacles in which thecentral portion of the bottom is pushed back inwards, by means of amoving mold bottom, which deforms, by punching ("punting") the bottom ofthe hot-shrunk blank, which blank is shorter than the length of thefinal receptacle to be obtained, but longer than the distance betweenthe orifice of the bottle neck and the top of the convex bottom of thefinal receptacle. In addition, operating that process of "punting" thebottom of the shrunk blank, and then blowing to form the finalreceptacle requires the presence of a rod inside the blank for centeringit and holding it in place.

As a result, the known method described in that document is not simpleto implement, mechanically speaking, and it is restricted tomanufacturing receptacles having a re-entrant bottom, and that is notalways required by users.

SUMMARY OF THE INVENTION

An essential object of the invention is thus to propose an improvedmethod and installation making it possible to obtain a receptacle havinga bottom that withstands temperature well and made of a material that isno longer significantly subject to induced stress, so that it cansubsequently be subjected to considerable heating such as that inducedby hot-filling, without suffering perceptible deformation, said methodand installation being as simple as possible to implement on the basisof the method and installation described in document FR-A-2 658 119(EP-A-O 442 836).

To these ends, in a first aspect, the invention provides a method asspecified in the preamble, which, when implemented in accordance withthe invention, is essentially characterized in that:

a/ the intermediate receptacle is used to produce a blank having ahot-shrunk body of predetermined length greater than the length of thebody of the final receptacle to be obtained;

b/ then the blank having a hot-shrunk body is subjected to preliminaryblow-molding to form the body of the final receptacle to be obtained,with the exception of its bottom zone, the length of the receptaclehaving an un-molded bottom zone as obtained in this way beingsubstantially equal to the length of the final receptacle to beobtained; and

c/ final blow-molding of the receptacle having an unmolded bottom zoneis performed, during which the body and the bottom zone are given theirfinal shapes and dimensions, thereby obtaining the final receptacle.

During the preliminary molding of the hot-shrunk blank, which isadvantageously performed by low pressure blowing (e.g. in the range 10⁵Pa to 1.5×10⁶ Pa, the height of the blank decreases because of theexpansion of the shoulder and the body which come into contact with theside walls of the mold cavity. However, the low inflation pressure isinsufficient to stretch and lengthen the material constituting thebottom zone of said blank. This preliminary molding operation, inparticular by low pressure blowing, can be considered as changing theshape of the blank without changing its surface area. Under suchconditions, the bottom zone of the blank is not stretched, is notsubjected to a decrease in thickness, and its material is not the seatof any induced stress.

Because the receptacle having an unformed bottom as obtained in this wayis of a length that is substantially equal to the length of the finalreceptacle to be obtained, the non-formed bottom of said receptacle isin contact with or nearly in contact with the bottom of the mold cavityin which the final molding is performed which is advantageouslyperformed by high pressure blowing (e.g. 10⁶ Pa to 4×10⁶ Pa). As aresult, the bottom of the receptacle is stretched transversely only andis subjected to no or nearly no longitudinal stretching. Also, since thediameter of the bottom of the hot-shrunk blank, and then of thereceptacle having a non-formed bottom is only slightly smaller than thediameter of the mold cavity, the thickness of the bottom zone is notsignificantly reduced and no significant stress is induced, inparticular in the bottom zone. A final receptacle is thus obtained whichis suitable for withstanding subsequent heating, such as duringhot-filling, without deforming.

In addition, the method is performed without great departure from themethod described in documents FR-A-2 658 119 (EP-A-O 442 836), such thatimplementing it under the conditions explained below requires only minoradaptation to the molding installation and its operating conditions.

In an advantageous preferred implementation of the invention, provisionis made for the preliminary and final molding operations to be preformedin the same mold which is provided with a moving bottom that is suitablefor taking up two positions, namely:

a low position for preliminary molding in which the length of the moldcavity is greater than the length of the body of the final receptacleand the mold cavity is capable of receiving the above-mentioned blankhaving a hot-shrunk body; and

a high position for final molding, in which the length of the moldcavity is the same as the length of the body of the final receptacle;

and the mold bottom is moved from its low position to its high positionduring or after formation of the receptacle having an unmolded bottomzone in such a manner that it exerts substantially no thrust against thebottom of the receptacle having a non-molded bottom zone.

It is emphasized at this point that the moving bottom of the mold, whileit is rising, does not push against the bottom of the receptacle havingan unformed bottom, and it therefore exerts no "punting" action on saidbottom, in contrast to the disclosure of document EP-A-O 559 103. Themobility imparted to the mold bottom therefore serves solely to adaptthe volume of the mold cavity very closely to the actual volume of theobject to be molded while the process is taking place.

In a second aspect, the invention provides an installation forimplementing the above method, which installation, when embodied inaccordance with the invention, is essentially characterized:

in that said means for manufacturing a receptacle blank having ahot-shrunk body are organized for producing a blank whose shrunk body islonger than the body of the final receptacle to be obtained;

in that the molding means comprise a mold matrix having a moving bottomdisplaceable between two positions, namely a low position forpreliminary molding in which the matrix defines a mold cavity having thesame transverse dimensions as the body of the final receptacle to beobtained, but having longer longitudinal dimensions, and a high positionfor final molding in which the matrix defines a mold cavity having thesame dimensions both transversely and longitudinally as the body of thefinal receptacle to be obtained; and

in that said installation further includes:

low pressure blowing means for blowing the blank having a hot-shrunkbody disposed in the mold cavity when the bottom of the mold matrix isin said low position, to form a receptacle having a body that is molded,with the exception of its bottom zone, and having a length that issubstantially equal to the length of the final receptacle to beobtained;

high pressure blowing means for blowing the receptacle having thenon-formed body disposed in the mold cavity while the bottom of the moldmatrix is in said high position, to form the final receptacle that is tobe obtained; and

means for controlling displacement of the moving bottom of the matrixfrom the low position to its high position during or after formation ofthe body, with the exception of the bottom zone, of the receptaclehaving an unformed bottom, without the moving bottom of the moldexerting perceptible thrust against the bottom of said receptacle havingan unformed bottom.

Preferably, the low pressure blowing means are organized to blow at apressure lying in the range 10⁵ Pa to 1.5×10⁶ Pa, approximately, and thehigh pressure blowing means are organized to blow at a pressure lying inthe range 10⁶ Pa to 4×10⁶, approximately.

In a preferred embodiment, the moving bottom of the mold is secured tothe piston of a pneumatic actuator subjected to the blowing highpressure for displacement from its said low position to its said highposition.

Advantageously, the moving bottom has a duct passing therethrough andopening out into the mold cavity via at least one orifice that isapproximately central and that is connected to the low pressure tofacilitate unmolding of the bottom of the final receptacle.

The invention will be better understood on reading the followingdetailed description in which reference is made to FIGS. 2 and 3 of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are views of a mold cavity showing known blow-moldingprocess stages, including a heat-shrunk blank having dimensions smallerthan the mold cavity, a blank at a subsequent stage, and the receptacleat its final dimensions, respectively.

FIGS. 2A to 2D are highly diagrammatic views of a mold in aninstallation organized in accordance with the invention and respectivelyshowing the main successive steps in forming a receptacle from ahot-shrunk blank; and

FIG. 3 shows in highly diagrammatic manner the shapes of receptaclesthat correspond to the main steps of the method implemented.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference initially to FIG. 3, the initial preform 1 is molded inknown manner, by blowing or by stretching-blowing in a mold, having sidewalls at a temperature lying in the range about 5° C. to about 90° C.and having an end wall at a temperature lying in the range about 5° C.to about 40° C. for a preform made of polyethyleneterephthalate (PET) toproduce an intermediate receptacle that is both longitudinally anddiametrically over-dimensioned. Thereafter, the over-dimensionedreceptacle 2 is heated to release the stresses induced by thestretching-blowing, thereby providing a hot blank 10 having a body thatis shrunken both longitudinally and transversely. The above steps areperformed under the general conditions specified for that purpose indocument FR-A-2 658 119 (EP-A-O 442 836), with the exception thatoperating conditions need to be adapted so that said shrunken blank 10is longer than the length of the final receptacle to be obtained.Various solutions all within the traditional knowledge of the personskilled in the art can be used and/or combined to achieve the desiredresults, and the person skilled in the art is thus entirely capable ofspecifying the mechanical and/or thermal operations that are appropriateas a function of the material used and the dimensions of the finalreceptacle.

The hot-shrunk blank 10 is then heated to a temperature lying in therange 150° C. to 240° C., and processed by blowing in a mold 11 having awall temperature lying in the range 110° C. and 160° C. As shown inFIGS. 2A to 2D, the blow mold is of the three-part type, having a righthalf-mold 11a, a left half-mold 11b, and a moving bottom 12. Togetherthe two half-molds 11a and 11b define a cavity 13 having the shape ofthe body of the final receptacle that is to be obtained.

The moving bottom 12 is secured to a piston 14 which is associated inmoving and sealed manner with a chamber 15 that is defined in the base16 of the mold. The base is itself a moving base, being movable alongthe longitudinal axis of the mold cavity, between a high position and alow position. In the high position, the base is enclosed between thebottom portions of the half-molds 11a and 11b and forms part of the moldcavity. The low position enables the molded receptacle to be unmolded.The actuator constituted in this way is preferably of the pneumatic typeso as to be capable of being actuated directly under conditions that areexplained below by the fluid under pressure (compressed air) used forblowing the receptacle. The pneumatic actuator is also of thedouble-acting type and two ducts 17 and 18 pass through the base 16 toopen out into the chamber 15 on opposite sides of the piston 14.Finally, to facilitate unmolding of the finished final receptacle, themoving bottom 12 has a duct 19 passing therethrough, in particularaxially, that opens out into the molding surface 20 of the bottom via atleast one substantially central orifice 21, and preferably via aplurality of orifices as shown in FIGS. 2A to 2D. The duct 19 conveysfluid under pressure for unmolding, which fluid may, here again, beconstituted by the fluid under pressure (compressed air) used forcontrolling displacement of the actuator to cause it to move back downtowards a low position: to this end, the duct 19 is in communicationwith the chamber 15 adjacent to the top face (in the drawings) of thepiston 14, into which the duct 18 also opens out.

The installation operates as follows.

The hot (150° C.-240° C.) shrunk blank 10 is installed in the mold 11while the moving bottom 12 is in its low position (in the drawing) andleaves a mold cavity of maximum length; the surface 20 of the movingbottom 12 is flush with the bottom of the blank 10 but does not applyforce thereto (FIG. 2A).

Preliminary molding is then performed (FIG. 2B) by blowing a fluid(compressed air) into the blank 10 at a relatively low pressure--e.g.10⁵ Pa to 10⁶ Pa--and for a short length of time--e.g. 0.2 s to 0.6s--as represented by arrow Pps in FIG. 2B.

During this step, the blank looses height due to inflation of itsshoulder and of its body which comes into contact with the side walls ofthe mold cavity 13. However, at this stage, the blow pressure isinsufficient to stretch and lengthen the material of the blank, and inparticular in the bottom zone thereof. This first blowing or"pre-blowing" changes the shape of the blank while leaving its initialsurface area unaltered and thus conserving wall thickness in all zonesof the blank.

At this stage it may be observed that the blank or receptacle having anunformed bottom 22 (see FIG. 2B) has shrunk longitudinally so as topresent substantially the same length as the final receptacle which isto be obtained, and that its bottom is now situated inside the moldcavity very close to the level which is going to be occupied by thebottom of the final receptacle.

The moving bottom 12 of the mold is then raised to bring it to its highposition, as shown in FIG. 2C, by applying fluid under pressure againstthe piston 14 via the duct 17, as represented by arrow Ps in FIG. 2C. Inpractice, the same high pressure compressed air is used as is going tobe used for final blowing of the receptacle, as described below. In itshigh position, the moving bottom 12 is situated right against the bottomof the receptacle having an unformed bottom 22, or in the immediatevicinity thereof, while nevertheless not pushing back the bottom of thereceptacle so as to avoid exerting any force thereon. At this stage, themold cavity has the shape and the dimensions, and in particular thelength, of the final receptacle that is to be obtained. Thus, the bottom14 of the mold is moved from its low position to its high position tocompensate for the reduction in length of the receptacle.

It will be observed at this point that the moving bottom 14 may be movedsimultaneously with the pre-blowing operation taking place while theblank 22 is shrinking longitudinally, so that the moving bottom 14actually follows the rising bottom of the blank but without applyingforce thereto, thus enabling cycle duration to be reduced.

Finally, a final blowing operation (arrow Ps) is performed under highpressure--e.g. 10⁶ Pa to 4×10⁶ Pa--in the receptacle 22 having anunformed bottom, so as to obtain the desired final receptacle 23 (seeFIG. 2D). Given the shape of the receptacle 22 having an unformed bottomwithin the mold cavity relative to the bottom 12 of the mold in its highposition (FIG. 2C), the bottom of the receptacle is subjected todiametral stretching only, with axial stretching being non-existent.Since the diameter of the bottom of the receptacle 22 having an unformedbottom is, in addition, only slightly smaller than the diameter of thebottom of the mold cavity, diametral stretching remains small inamplitude: as a result, the thickness of the material in the bottom zoneis changed little and the stresses induced therein remain small. Thefinal receptacle 23 is therefore suitable for being subjected,subsequently, to high temperatures (e.g. during hot-filling), withoutpresenting significant deformation.

After final blowing has been performed, air under low pressure, 5 to6×10⁵ Pa, is blown via the conduit 18 while the moving bottom 12 ismaintained in its high position by air under high pressure blown via theduct 17. The air under low pressure passes via the duct 19 to theorifice(s) 21 in the surface 20 of the moving bottom 12, therebyseparating (unsticking) the bottom of the molded receptacle from themoving bottom.

Then, the high pressure air feed to the duct 17 is disconnected whilethe low pressure air feed is maintained to the duct 18, thereby bringingthe moving bottom 12 back down to its low position.

Finally, the two half-molds 11a and 11b are separated, the moving base16 is lowered to its low position, and the receptacle 23 is unmolded.

Naturally, and as can be seen from the above, the invention is notlimited in any way to its applications and embodiments that are moreparticularly envisaged; on the contrary, it extends to any variantsthereof.

What is claimed is:
 1. An installation for manufacturing a receptaclefrom a preform of thermoplastic material, the receptacle being suitablefor being subsequently subjected, without significant deformation, torelatively severe temperature conditions encountered during hot-fillingor pasteurization processes, the installation including means formanufacturing a receptacle blank having a hot-shrunk body from thepreform, and molding means for forming a final receptacle from theblank, the body of the final receptacle having its final shape anddimensions, wherein:said means for manufacturing a receptacle blankbeing adapted to form a blank whose shrunk body is longer than the bodyof the final receptacle to be obtained; the molding means comprise amold matrix having a moveable bottom portion displaceable between afirst lower position for preliminary molding in which the matrix definesa mold cavity having the same transverse dimensions as the body of thefinal receptacle to be obtained, but having longer longitudinaldimensions, and a second higher position for final molding in which thematrix defines a mold cavity having the same dimensions bothtransversely and longitudinally as the body and bottom zone of the finalreceptacle to be obtained; low pressure blowing means for blowing theblank disposed in the mold cavity when the moveable bottom portion is insaid first position; high pressure blowing means for blowing thereceptacle disposed in the mold cavity when the moveable bottom portionis in said second position; and means for controlling displacement ofthe moveable bottom portion from the first position to the secondposition without exerting perceptible thrust against the bottom of saidreceptacle having an unmolded bottom zone.
 2. An installation accordingto claim 1, wherein the low pressure blowing means are adapted to blowat a pressure of from approximately 10⁵ Pa to 1.5×10⁶ Pa, and the highpressure blowing means are adapted to blow at a pressure of fromapproximately 10⁶ Pa to 4×10⁶ Pa.
 3. An installation according to claim1, wherein the moveable bottom portion is secured to a piston of apneumatic actuator subjected to the blowing high pressure fordisplacement from its said first position to its said second position.4. An installation according to claim 1, wherein the moving bottom has aduct passing therethrough and opening into the mold cavity via at leastone orifice that is approximately central and that is connected to thelow pressure means to facilitate unmolding of the bottom of the finalreceptacle.